Chlorine sensors are well known in the art.
A first type of chlorine sensor, disclosed for example in U.S. Pat. No. 5,725,747, uses a reagent liquid in which some metallic electrodes are immersed. The first drawback of this type of chlorine sensor is that, due to the presence of a reagent liquid that could contaminate the fluid whose chlorine amount is to measure, they are not suitable when the fluid is drinking water. Additionally, the fluid can dilute the reagent, reducing the lifetime without maintenance. Moreover, they require a periodical maintenance, during which the reagent liquid shall be changed. This maintenance operation has to be performed every two months and has to be made by a technical specialist, since complex operations shall be performed and since the sensor shall be recalibrated. This results in high operating costs.
The document U.S. Pat. No. 5,939,399 discloses an amperometric electrochemical sensor that does not use any reagent liquid. It comprises a semi-conductor sensing element, a measuring circuit and housing. The semi-conductor sensing element has a planar structure and comprises an insulating substrate, a set of planar complete electrodes (one working electrode, one counter electrode and one reference electrode), a diffusion membrane, which is deposited on the working electrode, and connections means to connect the electrodes to the measuring circuit. The semi-conductor sensing element is micro fabricated, being obtained by photolithographic techniques.
At least the semi-conductor sensing element is immersed in a fluid, for example in a pipe. The sensing element constitutes an electrochemical microcell, the electrochemical reaction with the fluid producing an electrochemical current, depending on the amount of chlorine in the fluid.
This type of chlorine sensors has good performances and is also suitable to be used in drinking water, but its constitution is such that the sensing element shall be replaced periodically, approximately once a year. This time limitation is mainly due to the delaminating of the diffusion membrane due to bad adhesion on the working electrode, as well as insufficient packaging, allowing the fluid to penetrate within the probe, creating short-circuit. Compared to a sensor using reagent liquid, the operating costs due to maintenance are reduced, since it occurs less frequently, and since the operation can be performed by a non-qualified person. However, as it is not possible to disassemble the sensor, when the sensing element replacement has to be performed, the whole sensor shall be replaced, i.e. the sensing element, the measuring circuit and the housing. Operating costs remain high.
In addition, the sensing element is placed on a support plate, and this support plate is oriented vertically with respect to the fluid stream. This orientation can be disadvantageous since the fluid could contain some small air bubbles. With a vertical orientation of the plate, it is possible that some air bubbles stay hanged on the sensing element, leading to a mismeasurement since the sensing element is no more in contact with the fluid.
Additionally, the support plate is prominent. This disposition has different drawbacks. The first drawback is that, if some small objects or particles circulate in the fluid, they could damage or even destroy the sensing element if they hit it. The second drawback is that the strain on the plate can be high, in particular when the flow of the fluid is high. The third drawback is that the sensing module is somehow fragile when handling it, the sensing element being directly exposed. The fourth is the limited lifetime of the membrane adhesion leading to bad definition of diffusion layer and change in the sensitivity.
The present invention proposes an electrochemical sensing probe such that these drawbacks are avoided.